The present invention relates to a method for shot blasting objects for the purpose of removing coatings of paint, varnish or like substances therefrom, in which said objects are cooled in a condensed gas, for instance liquid nitrogen, and thereafter subjected, in a chilled state, to the effect of a blasting agent, such as steel shot, pellets or like particles delivered from a shot blasting machine, and in which method coating fragments and blasting shot are collected and separated from one another and the shot returned to the blasting machine. Such paint removing methods are used, for example, in the car manufacturing industry, in which painting or varnishing of car bodies is carried out with the car bodies suspended from fixtures intended for this purpose. As the car bodies are sprayed, the fixtures become coated progressively with successive layers of paint or varnish, which finally have to be removed from the fixtures. Removal of these paint or varnish layers is effected by immersing the fixtures into a liquid nitrogen bath, until the fixtures are sufficiently chilled, whereafter the fixtures are blasted with steel shot or some like blasting agent, delivered by a blasting machine.
Blasting is effected, by throwing or slinging the shot against the paint coating on the fixtures, said coating having become brittle as a result of its exposure to the low bath temperature. Contact of the shot with the paint layers causes the paint to break up into fragments and dust, which fall to the floor together with the shot. It is then necessary to separate the shot from the paint debris. This is achieved by transporting the shot and paint debris to a drum sieve, in which larger paint fragments are removed, and then to a wind sieve, in which paint dust and shot fragments are separated from useful blasting shot. The blasting shot is transported from the wind sieve to the shot magazine of the blasting apparatus, from where the shot is conveyed to the throwing wheel, operative to throw the shot against the fixtures.
However, in addition to coatings of paint and similar coating substances, these fixtures also become coated with anti-rust oils and greases, or other types of oil and greases, which when the fixtures are immersed in liquid nitrogen become brittle and hard. When the fixtures are subsequently shot blasted, to remove the paint and varnish coatings, the oil loosens together with the paint and softens or melts such as to form together with paint dust and blasting shot a heavy dough-like mass which tends to fasten in the shot transporting and separating equipment, therewith creating disturbances in the operation thereof.
Furthermore, the oil which accompanies the shot to the shot magazine of said blasting machine causes the shot to pack into adhesively bonded aggregates. These aggregates, or lumps of shot, interrupt the smooth delivery of the shot to the throwing wheel of the blasting machine, therewith varying the efficiency of the blasting operation. Furthermore, interruptions in operation are often caused by blockaging of the blasting shot circulating system.